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Topic: VTA ST-70 (Read 4912 times)

Well, my best friend is FINALLY getting into 2 channel, specifically tube gear so, I'm building a Bob Latino ST-70 for him with all the fixins. Upgraded caps, matched quad and stepped attenuator. Haven't built anything in 2 years and getting the itch.

Well, I previously built one for myself but with some slight modifications to the one I will start tonight. Mine has Gelelex KT-66's and Mundorf SGO caps. This one will be EL-34 and K42-Y PIO caps. I compare now with my OTL and I really like the ST-70 amp but with my Zu Druids, I prefer the OTL and with my Paradigm's I prefer the ST-70! Maybe something to do with speaker crossovers or just the difference with SDFR speaker versus traditional 3 way. Would like to hear comments on this as to why. I will compare my ST70 to the one I build and will report!

The output impedance of the two amps is different. OTL has a LOT of negative feedback which lowers the source impedance to like 0.1 ohms. That will really affect some speakers that are designed with solid state amp in mind, like Paradigm/Anthem.

Normal tube amps usually have less feedback, higher source impedance and weaker damping. Some people like this as warmer, softer sound. SDFR speakers sound better with less electrical damping. Nelson Pass wrote an article on First Watt website about this, talking about his current source amps, which have no electrical damping at all. http://www.firstwatt.com/pdf/art_cs_amps.pdfThere may be other articles there, this is the first hit I found on goog.

When I was building boxes for my featrex D5nf FR drivers I built a LM1875-based current source chip amp which made the SDFR sound very good. Stopped all the shouting they tend to do. But the 1.5ohm source impedance tube amp Manley Snappers were even better. But Cary SLI-80 was atrocious as were many normal voltage source solid state amps. It's a weird thing.

I don't think it's weird at all. It is just more complicated that most people realize or at least are willing to admit.

Over my years in this hobby I have dealt with many measurement objectivists who would say that there was no difference between two components because the measured the same. And when a while bunch of us listened and heard the same differences we were told it was all in our heads because the components "couldn't" sound different.

My answer to that was always if we hear a difference that you can't measure it the answer is not that the difference does not exist but rather that you are measuring the wrong things.

Astute explanation for sure. Finished the build and bias all dialed in. Initially EL-34 sound has decent low end on my spare Paradigm Studio 10V.5's. Once I get some hours and this thing opens up a bit, will compare to the same amp I built 3 years ago with KT-66's. Nice build and took my mind off a crappy work week last week!

So with about 25 hours of break-in, amp sounds pretty decent. Will change the rectifier to a Mullard GZ34 since I've read they tend to last much longer than the Chinese one with the kit. Zu Druids sound pretty nice. Bias is staying quite steady with multiple power ups. Now the recipient of this beauty is looking for some speakers. Local shop in Chi-town (Holm audio) has some nice trade-ins. Will bring the amp in to audition a few sets. Recommendations are always welcome. Choices - LSA, Monitor audio, Vienna Acoustics and saw nice pair of Cornwalls for $1,200 (may be too large).

The DIY speaker from Bob Brines using TangBand W8-1772 driver is pretty good if funds are limited. But it's single driver, so the high treble beams strongly. I loved the sound of the floorstanding model but couldn't stand the beaming. http://brinesacoustics.com/Pages/TangBand/Main.html